This invention is directed to a reinforced flexible duct which has a continuous liner, as well as the machine and method for making the flexible duct.
There is a considerable body of prior art in flexible ducting or tubing. Single- or double-ply tubes are fabricated of a precoated fabric by winding, together with a wire helix. The precoated fabric is helically bonded to itself by solvent or adhesive bonding into one integral tube. Single-ply tubing is helically overlapped a single pitch, and the wire is wound into that overlap area to be captured between the coated fabric layers. A similar prior art construction is formed with a wider fabric which is almost two pitches wide so that two layers of fabric over or underlie the wire helix. Nearly the entire tube has three layers of fabric by this construction. Again, solvent or adhesive bonding is employed. In both of these structures, the coating on the fabric is a polymer composition material, such as polyvinyl chloride, which can be solvent-activated to adhere doing helical winding. The fabric is generally nylon, rayon, dacron, cotton, or fiberglass.
In another single-ply construction, during the winding the wire helix is encased with the single ply of fabric. This is accomplished by folding the fabric over the helical wire so that both edges extend in the same direction. Helical winding in the single-ply mode causes the area over the wire helix to have two layers thereon, with one fabric layer at some areas therebetween. This type of construction can also be formed in double-ply tubing by providing a wider fabric layer, one of which is almost two pitches wide, so that three layers of fabric are wound over the wire helix. This type of tubing construction also requires bonding with a compatible solvent or adhesive. The coating on the fabric can be either thermoplastic or elastomeric. Each of these structures has the disadvantage that it has a lap on the internal surface which is exposed to the fluids or solids passing through the duct. Thus, the duct must be installed with the overlap going with the flow of materials being handled. These laps increases air friction due to the exposed overlap edge. The chemical bonding, either by adhesive or solvent bonding, requires approximately 7 days for the solvent to completely disappear or the adhesive to fully cure so that full peel adhesion strength is achieved. Furthermore, chemicals wick through the fabric overlap to result in delamination and failure of the duct. The duct has poor abrasion resistance and cannot be used for liquid handling. There is a poor leakage rate due to the permeability of the material. Thus, there are considerable disadvantages to this design.
Another prior art design features sewn construction with a plastic liner being stitched to a single-ply, coated fabric layer with the helical wire captured between two rows of stitching. This construction has the disadvantage that the liner flaps are in the air flow and thus can be installed in only a certain way so that the liner flaps are directed downstream. This requires that the duct be marked on the outside. The construction is such that the duct has a poor bend radius and poor flexibility. There is poor external abrasion resistance, poor tensile strength, poor air friction loss, and since it is made by hand, it is expensive. Furthermore, it can be manufactured only in limited lengths. Thus, the sewn type of prior art construction is also disadvantageous so that there is a need for a reinforced flexible duct with integral molded liner.